1. Field of the Invention
The control of sand production in a well is a problem that has been with the oil industry for a very long time. Experience indicates that sanding problems are directly related to production rates of the well. Therefore, the problem of sanding has become more critical lately since proration control by the government has been substantially eliminated due to shortages. Consequently, the oil companies are concerned with substantially increasing production of their wells. In fact, the government is actually requiring an increase of production in Federal waters. It is therefore apparent that oil producing companies would like to know the maximum rate at which they can produce without sanding from wells in which no consolidation or gravelpack has been used. Furthermore, oil companies would be very interested in knowing which wells require gravelpacking or consolidation from the very beginning of production, so that control measures can be instituted from the start. Since there are friable sand reservoirs in known sand producing areas from which economically attractive production rates may be obtained without the use of any form of sand control, it is desirable that some means for distinguishing or predicting those competent sand formations from incompetent sand formations be available so that sand controlling techniques such as gravelpacking or plasticizing may be avoided when unnecessary, or, on the other hand, may be resorted to before sanding problems develop from over production in those formations where such sand control is necessary.
2. Description of the Prior Art
In seeking information concerning zones bearing hydrocarbons such as oil and gas that may exist in subsurface earth formations adjacent a borehole drilled into these formations, various types of exploring devices are typically lowered into the borehole for measuring selective properties of the formations adjacent the borehole. Three principal types of such exploring devices are (a) electrical exploring devices (using either electrodes or induction coils), (b) sonic exploring devices, and (c) radioactivity exploring devices.
The electrical exploring devices measure the electrical resistivity or conductivity of the earth formations. Such electrical resistivity is determined primarily by the amount, distribution and resistivity of the fluids contained in the formation core spaces. The sonic exploring devices, on the other hand, measure the time required for sonic waves to travel across a given span of the earth formation which is related to the sonic velocity of the formation. This sonic velocity is determined primarily by the nature of the rock matrix and particularly its porosity, the state of confining stress and the type of fluid in the pore space.
The radioactivity exploring devices measure either the natural radioactivity of the formation or the radioactivity induced therein by bombardment of the formation with radioactivity particles or rays. Two particular radioactivity exploring devices used to investigate formations are the formations density logging tool and the neutron logging tool. The formation density logging tool emits gamma rays which are diffused through the formation and the number of diffused gamma rays reaching one or more nearby detectors are counted to provide a measure of the electron density of the adjacent formation. Moreover, it is known that this electron density is very closely proportional to the bulk density of the formation in substantially all cases.
The neutron tool, on the other hand, utilizes a source for emitting neutrons into adjacent formations. In one form of neutron device, these neutrons lose energy by collision with atoms of the formation. When the energy level of these neutrons is reduced to the epithermal energy range, they can be detected by a nearby detector which counts the number of epithermal neutrons. Since hydrogen atoms are the only ones whose weights are almost equal to that of the neutron, they are the most effective in reducing the energy level of the neutrons to enable their capture. Thus, it can be said that this type of neutron log is essentially a record of the hydrogen atom density of the rocks surrounding the borehole. Since the formation pore spaces are generally filled with water or fluid hydrocarbon, both of which have about the same amount of hydrogen, a neutron log does not distinghish between oil and water, but is primarily affected by the formation porosity. Gas, on the other hand, will alter this porosity determination by the neutron log.
In general, none of the electrical, acoustic, or radioactivity measurements taken alone give all of the required information concerning the hydrocarbons in the formations or the characteristics of those formations. The various factors which affect each measurement are taken into account and then an interpretation or deduction is made as to the probable characteristics of the formations.
There is considerable experimental evidence which indicates that there is a correlation between the intrinsic strength of a formation and the dynamic elastic constants of the formation as determined from sonic velocity and density measurements. One technique which attempts to predict the competency of sand and thereby also predict the maximum rate at which a well may be produced is described in a paper, "Estimation of Maximum Production Rates from Friable Sandstones Without Using Sand Control Measures", by N. Stein and V. W. Hilchie, Paper No. SPE 3499 published by the American Institute of Mining, Metallurgical and Petroleum Engineers Inc. Copyright 1971. According to this paper, the shear modulus is the most important elastic constant for predicting sanding problems, however, the technique described in the paper is based on the assmmption that the bulk modulus is constant throughout the formation. In general, the bulk modulus varies throughout the formation and this technique would not provide accurate results.
It is possible to obtain the mechanical properties or elastic constants of the formation, such as the shear modulus and bulk modulus (or bulk compressibility) from the value of Poisson's Ratio. Heretofore Poisson's ratio was determined from the sonic shear and compressional velocities, while the sonic compressional velocity which is generally referred to as the acoustic travel time of the formation is readily measured as described above, the sonic shear velocity, is highly attenuated. The sonic shear velocity approaches the velocity of the fluid in the formation, and shear arrivals are often masked in the sonic wave trains making it extremely difficult to measure.
Clearly a need exists for readily determining values for Poisson's ratio which can then be used to determine the mechanical properties and strength of a formation.